This invention relates generally to spread spectrum, code division/multiple access (CDMA) wireless communications systems and, more particularly, to method and apparatus for optimizing the allocation of spreading codes to mobile subscriber terminals.
Next generation cellular telecommunications systems (i.e., so-called third generation (3G) systems), such as the Universal Mobile Telephone System (UMTS) in Europe and CDMA2000 system in the United States, will be based on wide bandwidth CDMA technology. These systems are expected to enable the simultaneous transmission of real-time video streams, packet mode data and/or speech in the same cell and on the same frequency channel.
The current second generation systems were originally designed for low bit rate circuit switched services (speech). In one well known second generation CDMA system (TIA/EIA IS-95) there is only one, relatively long spreading factor that is used. That is, while there are many spreading codes in use, there is only one spreading factor or code length.
However, for the proposed third generation systems the implementation of high bit rate services will require that shorter spreading codes be used (e.g., spreading sequences as short as four chips). An alternate method for implementing high bit rate services is to employ multi-code transmissions, wherein a plurality of logical links and spreading codes are allocated for the same connection (user and service), and wherein the data stream is split and transmitted in parallel over the plurality of logical links. In some cases both methods can be used together (i.e., short spreading sequences and multiple parallel links.)
In theory, both of these methods provide nearly identical spectrum efficiency and link performance. However, the first method (i.e., shorter spreading codes or decreased spreading factor) is preferable from the mobile station (MS) point of view, as the complexity and the processing requirements, and thus the current consumption, are reduced as compared to the use of multiple links and spreading codes for one connection.
One major disadvantage to the use of the shorter spreading codes (decreased spreading factor) is that as the spreading codes are made shorter, their performance in an actual multipath channel is decreased. This is due primarily to poor auto-correlation and cross-correlation characteristics of short spreading codes, as compared to longer spreading codes.
FIG. 1A is a graph that illustrates the results of a simulation of the effect of unequal and non-optimized inter-code interference in a wide bandwidth CDMA communication system. As can be seen, the performance of different links can vary dramatically. This is due to dissimilar spreading code correlation characteristics in a multipath environment. The codes used in this simulation were derived from an orthogonal Walsh tree.
This problem was mitigated in the currently deployed second generation CDMA systems by the use of long scrambling codes (as opposed to spreading codes) to average cross-correlation effects between the spreading codes.
It should be noted that in the presently envisioned third generation CDMA systems the use of long scrambling codes has also been proposed so as to average the interferences resulting from dissimilar spreading code correlation characteristics in a multipath environment.
FIG. 1B is a graph showing cross-correlation functions for short (8-bit) Walsh spreading codes as a function of code delay, wherein for clarity only four of a possible 56 cross-correlation functions are depicted, one for good, two for moderate, and one for poor cross-correlation results.
FIG. 2 illustrates the cross-correlation results for an eight chip long Walsh code without the use of a long scrambling code (delay range seven chips), while FIG. 3 shows the significant averaging in interference that results when the long scrambling code is added.
It can be appreciated that in the second generation CDMA communications systems the use of long scrambling codes was adequate to overcome the cross-correlation effects of short spreading codes in a multipath environment. However, the second generation CDMA systems are essentially symmetrical service systems optimized for speech transmissions, wherein the same bit rate is used in the uplink (mobile station to base station) and in the downlink (base station to mobile station) directions. In such symmetrical systems the major system capacity limitation has traditionally been limited by the uplink, where errors in the mobile station power control has been the primary problem. However, in the proposed third generation systems the downlink will be managed separately from the uplink, as many data services are very unsymmetrical. For example, a user performing WWW-browsing may have a very low bit rate requirement for the uplink (e.g., one sufficient for handling only low speed manual character input) , while the downlink may have a very high bit rate requirement (e.g., if the user has requested a WWW page containing graphics and/or video and audio content).
Referring to FIG. 4, in the conventional second generation CDMA systems the spreading codes are typically allocated to users in a random or xe2x80x98naturalxe2x80x99 order. In the natural order allocation technique, the spreading codes can simply be allocated in first-free-location-found order from a table containing spreading codes. These spreading codes are typically stored in the table in the order in which they are output by a generator polynomial.
The inventor has recognized that should it be desired to not use the long scrambling code, which guarantees moderate performance for all spreading code combinations, then this conventional spreading code allocation technique will not be optimum for assigning spreading codes in the third generation (and higher) CDMA telecommunications systems.
It is a first object and advantage of this invention to provide an improved technique for allocating spreading codes to users.
It is another object and advantage of this invention to provide an optimized spreading code assignment technique in a CDMA system that does not employ scrambling codes.
The foregoing and other problems are overcome and the objects of the invention are realized by methods and apparatus in accordance with embodiments of this invention.
A method is disclosed for assigning spreading codes to mobile stations located within a cell, as is a CDMA communications system that operates in accordance with the method. The method includes steps of (A) assigning a set of spreading codes for use in a particular cell; (B) determining values of entries in a spreading code performance table based on a weighted combination of cross-correlation factors between individual ones of the spreading codes of the set of spreading codes, and also based on an auto-correlation factor of individual ones of the spreading codes of the set of spreading codes, within a delay spread range that is typical for the particular cell; and (C) assigning a spreading code from the performance table by selecting a spreading code that will cause a least amount of interference with a sub-set of already assigned spreading codes. Preferably the data to be transmitted is not scrambled with a scrambling code, but is only spread using the assigned spreading code. The steps of determining and assigning can be performed by a Radio Resource (RR) Management software module or function that runs on a data processor of a base station controller.
The step of determining the values of the entries in the performance table may be further based on a consideration of a simultaneous presence of high data rate mobile stations, within the delay spread range that is typical for the particular cell. The step of determining can establish a fixed performance table or a dynamic performance table, wherein the values of the entries are updated in real-time or substantially real-time.
The spreading code can be assigned to a mobile station by a spreading code handover technique, wherein an already assigned spreading code is unassigned and replaced by a newly assigned spreading code. In this case a spreading code handover is performed upon an occurrence of a condition wherein an amount of performance improvement that will be experienced by replacing an already assigned spreading code with a new spreading code exceeds a threshold value.
Preferably the step of assigning assigns a spreading code that is selected to minimize an RMS correlation metric error value for all, already assigned spreading codes.